I've downloaded the schematics from the Ernie Ball site for the correct chassis number and will bias to the spec listed on that.

While I'm at it, is there any reason to replace the 12ax7? I'd like to keep as much of the amp as original as possible (pox on the person who broke the original RCA tubes) but if it will improve performance, I may as well.

You don't need a super high quality 12AX7 tube in
that slot. Call Lord Valve (who deals exclusively in
tube amps) for his best opinion. Or, just use your
generic groove tubes, just not cheap thin glass
Chinese. Power tubes, yes, very important

Great site indeed. Musicman amp owners are a
strong breed keeping these gems running after 30 years.
I also own and use a Blackface 1967 Fender Showman

make sure you adjust your bias. These are the best I've
played, and are pretty close to my beloved Sylvania 6CA7's which are hard to find and pricey

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The EH's ARE Sovtek, and JJ Electronics (formerly Tesla) does not and has not ever made a big bottle 6CA7.

Amp's need bias adjustment ANY time the output tubes are changed. .5 volts is the bias voltage for the model with the driver transistor, for the transistor itself, NOT the output tubes! At .5 volt bias, output tubes will burn up.

Some Music Man amps ran near Class B which dumps in excess of 700 volts on its output tubes, so good quality tubes are a MUST. If the amp has the 12AX7 driver, its quality WILL make an audible difference. I like using 5751's in driver positions as it gives the amp more clean headroom.

Fat Willie is pretty reputable. Bill Bolton likes to talk about things that he knows VERY little about as though he is an expert. Like most things in life, you can't believe everything you read on the internet and I encourage everyone to double-check a lot of the supposed "facts" floating around in cyberspace. There are LOTS of great tube resources I'd be glad to steer anyone towards. It takes a little reading and legwork, but the knowledge you gain is invaluable.

I have yet to EVER see a driver tube wear out without something else being wrong. If it does need replacing, you'll notice low volume with little to no headroom. I'd replace the outputs first and see if the driver needs swapping afterward. If the amp was working fine before, it should still be fine.

Got the tubes, installed them, set the bias per MM schematics (courtesy EB website) and the HD130 is up and running again. Sounds pretty good through my Aggie 410, even if it does break up sooner than I'm used to. Looking back on the thread I noticed this from PBG:

".5 volts is the bias voltage for the model with the driver transistor, for the transistor itself, NOT the output tubes! At .5 volt bias, output tubes will burn up."

As this does not jell with the schematics, I just wanted to get more info regarding this statement. IOW, what are the other options for bias settings that won't cause long term damage?

Got the tubes, installed them, set the bias per MM schematics (courtesy EB website) and the HD130 is up and running again. Sounds pretty good through my Aggie 410, even if it does break up sooner than I'm used to. Looking back on the thread I noticed this from PBG:

".5 volts is the bias voltage for the model with the driver transistor, for the transistor itself, NOT the output tubes! At .5 volt bias, output tubes will burn up."

As this does not jell with the schematics, I just wanted to get more info regarding this statement. IOW, what are the other options for bias settings that won't cause long term damage?

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If your tubes aren't glowing red, then it's not too low. I did some more checking into MM amps and their bias voltage reading reference isn't the actual bias voltage, but rather the stage regulator FOR the bias. You're taking the grid voltage reading, but the tubes are cathode driven, meaning the total bias current is varied by the tubes' cathodes, like a cathode bias, but the amount of initial grid voltage can be varied like a fixed bias amp.

I am confused.(...) Question is, is it for bass or guitars? Cos the thread here says for bass.

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No wonder you're confused,because,AFIK,most MM's are not specified to be either bass or guitar amps.There is a wide plethora of MM users out there who use their beloved MM with a bass but I have my own reserves about that.
First thing that comes in mind is the output transformer's size.Most MMs I got on repair had a,say,rather "normally-sized" OPTs for the B-class output stage,meaning that you could run a guitar through that amp BUT a bass guitar....hardly.Why?Because the lower the freq.a OPT has to "transform" (primary-to-secundary-wise),the sooner that OPT's lamination goes into magnetic saturation and this translates as a distortion.Of course,this is (highly) desirable with a guitar but not with a bass wich is supposed to remain clean.Just look at a beefy bass amp's OPT and compare it visually with that skinny MM...you'll get the idea.
Second,I really don't dig the preamp's sections and the tonestack with ICs as seen in MMs.Surely,the MM's tonestack values (freq.controls) are OK for a guitar but,being a bass player myself,I don't like at all a guitar amp's tonestack "versatility" with a bass.Sure,some guitar players swear for a bass amp's tonestack (Marshall developed from a Fender Bassman,remember?),but,for us,lowenders,it rarely functions this way.
HTH!
Regards,